Abstract. We study ergodic properties of Pauli-Fierz systems---$W^*$-dynamical
systems often used to describe the interaction of a small quantum system
with a bosonic free field at temperature $T\geq0$. We prove that,
for a small coupling constant uniform as the positive temperature
$T\downarrow 0$, a large class of Pauli-Fierz systems has the property
of return to equilibrium. Most of our arguments are general and deal with
mathematical theory of Pauli-Fierz systems for an arbitrary density
of bosonic field.